With the advance of robot technologies in recent years, robots have performed some complicated tasks, such as assembly of industrial products, instead of human beings. In order for a robot to hold and assemble a part with an end effector such as a hand, the measurement of relative positions/postures of the part to be hold and the robot (hand) may be required.
One method of measuring the position and posture of an object may use a feature detected from a gray image or color image (hereinafter, collectively called a two-dimensional image) photographed with a camera.
Alternatively, a three-dimensional form model of an object may be applied to distance data measured imagewise by a distance sensor. This scheme is called a model fitting.
In position and posture deduction by model fitting, the position and posture of some forms of objects may not be measured with high precision.
For example, the moments at the barycenter position and around the barycenter of an object which are calculated from distance data may be compared with a model. However, when the target object has an aspect ratio close to 1, it is difficult to deduct six parameters of a position and posture with high precision in a coordinate system of an image pickup apparatus.
PTL 1 discloses a method in model fitting to a depth map deduces position and posture parameters by a plurality of steps based on a coordinate system of an image pickup apparatus for higher stability and efficiency of the deduction.
PTL 1 discloses a method which deducts six degree-of-freedom components of a position and posture based on a coordinate system of an image pickup apparatus in order from a parameter which may be deducted with the highest precision and robustness. First, two parameters of rotation of an image coordinate axis of a captured image and one parameter of the position in the visual axis direction are deduced. Then, two parameters of the position perpendicular to the visual axis is deducted, one parameter of the rotation about the visual axis is deducted last. Dividing the parameter deduction procedure into steps allows measurement of the position and posture robustly and with high precision.
As pointed out in PTL 1, some objects may have degree-of-freedom components which are difficult to deduct for six degrees of freedom of a position and posture on the basis of an object (model) coordinate system. For example, some parts used in industrial products may have a nearly rotation symmetrical form and may have a minute notch for uniquely determining the posture component in the direction of rotation.
When the six parameters of the position and posture of such a part are attempted to deduct simultaneously by fitting between its image and a model, information lacks which contributes to the deduction of a specific degree of freedom component (in this case, the posture around the axis that defines the rotation symmetry of the model), compared with other degree-of-freedom components. This makes the deduction of the parameter in the direction of a specific degree of freedom difficult, resulting in low robustness and precision of the deducted position and posture.
PTL 1 discloses a method which deducts six degree-of-freedom components of a position and posture based on a coordinate system of an image pickup apparatus in order from a parameter which may be deducted with the highest precision and robustness. In other words, a specific degree-of-freedom component only is deducted separately on the basis of a model coordinate system. Except for the case where the rotation center axis of an object is matched with the visual axis of a used image pickup apparatus, the posture component around the rotation center axis of the object may not be deducted with high precision.